THE DISPERSAL OF SHELLS, 



CHAPTER I. 



FRESH-WATER SHELLS— ANOMALIES IN LOCAL 

 DISTRIBUTION. 



Most naturalists of recent years have agreed that " the 

 several species of the same genus, though now inhabiting 

 the most distant quarters of the world, must originally 

 have proceeded from the same source, as they are de- 

 scended from the same progenitor," and also, of course, 

 that " the individuals of the same species, though now 

 inhabiting distant and isolated regions, must have pro- 

 ceeded from one spot, where their parents were first 

 produced," ^ and the acceptance of this view, now that it 

 is very generally admitted that the great oceans — the 

 most formidable of all barriers limiting the migrations 

 of non-marine animals — have occupied the same general 

 positions as they do now throughout all known geological 

 periods,^ has naturally lent much interest and import- 



^ Darwin, " Origin," ed. 6, 1890, pp. 319-20. 



- Wallace, "Island Life," 1880, p. 144 ; ed, 2, 1892, pp. 149-50 ; 

 Darwin, " Origin," p. 288. For a summary of the evidence on this 

 head, see " Island Life,'' pp. 101-2 ; ed. 2, pp. 103-5. 



B 



